1. Technical Field
The present invention relates to a protection circuit for a subscriber telephone interface circuit, and particularly to an improved method and circuit for protecting the telephone interface from the application of excess voltages and currents.
2. Related Art
A subscriber line interface circuit (SLIC) is used in a telephone switching system to interface between the switching system and pairs of wires connected to a telephone terminal device.
In a conventional SLIC, a ringer detection circuit detects a ringing signal sent from a Public Switch Telephone Network (PSTN). The detection of ringing results in a speech circuit being connected across the line, and the ringing detection circuit is no longer required.
Current SLICs contain many sensitive electronic components, particularly in the speech circuit, which are easily damaged by excess voltage or current. The environment in which a SLIC is used means there are many circumstances in which such hazardous excess currents or voltages may be applied to a SLIC. Possible hazards include inappropriately connected power lines, or voltages induced from power lines running adjacent to the telephone line. SLICs are also subjected to rigorous overvoltage tests, during which excess voltages and currents are applied to the SLIC.
It is therefore necessary to protect the SLIC against damage that would otherwise be caused by these excess voltages and currents. It is important that any means of providing protection be cost effective, as the number of SLICs employed in a telephone switching system is very high, specifically, equal to the number of users.
It is common practice to insert a series resistor in the interface circuit to protect against lightning surges, and to conform to testing stipulated in FCC regulations. Unfortunately, it is difficult to set the value and power rating of this resistor as it must satisfy two conflicting requirements. Namely, it must be able to pass prolonged overvoltage tests stipulated in safety standards such as UL1459, while preventing fusing at the time of lightning surges in a controlled manner. This makes it difficult to meet conditions for the series resistor stipulated in the Recommended American Telephone equipment standards (EIA-470).
Many methods are known to offer the necessary protection. Earlier systems employed components such as relays, indictors, transformers, resistors, and fuses, etc., which can withstand stresses resulting from overvoltage. More modern systems, however, employ solid state components such as Zener diodes or thyristors, which are more susceptible to damage.
For instance, Japanese patent laid-open No. Sho. 63-304793 discloses a protection arrangement for a telephone subscriber line interface circuit connected via feed resistors to a telephone subscriber line comprising switching means, voltage sensing means, current sensing means, means for coupling the voltage sensing means to the line and for coupling the current sensing means to the feed resistors, whereby the voltage sensing means is responsive to voltage on the line and the current sensing means is responsive, when the switching means is closed, to current on the line, and control means for closing the switching means in normal operation and for opening the switching means in response to a voltage on the line determined by the voltage sensing means to exceed a predetermined threshold and in response to a current on the line determined by the current sensing means to exceed a predetermined threshold.
In addition, Japanese patent laid-open No. Hei. 11-164467 discloses an overcurrent protection unit which reliably protects a SLIC against overvoltage and overcurrent, provided with a function to detect the abnormal state, and which automatically resets when the overvoltage disappears.
However, both of these inventions employ dedicated components to detect the excess voltage or current state, which inevitably causes an increase in the number of components and thus cost.
The object of the present invention is therefore to make it possible for a subscriber line interface circuit to prevent heat radiation and ignition when an excessive voltage is applied, such as during testing to conform to American and Canadian safety standards (UL/CSA).
It is also an object of the present invention to provide this protection without the need for a dedicated detection circuit to thereby prevent any increase in the cost of the SLIC itself
This is achieved in the present invention by automatically disconnecting a low impedance speech circuit in such an event, so that only a high impedance detection circuit is placed across the line, thus limiting current. The present invention does this by using a detection circuit that is normally used during an on-hook state to detect ringing current indicative of an incoming call. After a speech circuit is switched in, this detection circuit functions to detect excessive voltage applied to the line during the call, and if excessive voltage is detected the speech circuit is disconnected.